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An optimum irrigation management strategy requires information on the yield loss associated with quantified field water deficits and the ability to assess the adequacy of irrigation intensity during the growing season. This study was conducted to develop a water‐production function for grape (Vitis vinifera L.) and to establish plantbased water‐status measurement techniques and critical values that assess irrigation regime adequacy. Field studies were conducted for 3 yr with ‘Thompson Seedless’ grapevine on Panoche clay loam (fine‐loamy, mixed [calcareous], thermic Typic Torriorthent) and Hanford sandy loam (coarse‐loamy, mixed, nonacid, thermic Typic Xerorthent) in the San Joaquin Valley of California. Differential irrigation quantities were supplied in several increments ranging from 40 to 120% of the expected nonstressed crop evapotranspiration (ETcrop). Midday measurements of leaf water potential (ΨL), water stress index (CWSI), and stomatal conductance (g1) closely associated with imposed water deficits and the resulting grape yields. All plant‐based water‐status‐reflecting measurements were highly correlated. Grape yields followed declining water availability essentially linearly over the range of available data. A 50% reduction in ETcrop resulted in a 26% yield loss. Irrigation intensities that resulted in ΨL > −0.9 MPa, CWSI < 0.2, and gs > 0.008 m s−1 maintained high grape productivity. These results can be used to profit‐maximize irrigation water as a production input.

The reproductive growth and water productivity (WP b ) of Thompson Seedless grapevines were measured as a function of applied water amounts at various fractions of measured grapevine ET c for a total of eight irrigation treatments. Shoots were harvested numerous times during the growing season to calculate water productivity. Berry weight was maximized at the 0.6-0.8 applied water treatments across years. As applied water amounts increased soluble solids decreased. Berry weight measured at veraison and harvest was a linear function of the mean midday leaf water potential measured between anthesis and veraison and anthesis and harvest, respectively. As applied water amounts increased up to the 0.6-0.8 irrigation treatments there was a signiWcant linear increase in yield. Yields at greater applied water amounts either leveled oV or decreased. The reduction in yield on either side of the yearly maximum was due to fewer numbers of clusters per vine. Maximum yield occurred at an ET c ranging from 550 to 700 mm. Yield per unit applied water and WP b increased as applied water decreased. The results from this study demonstrated that Thompson Seedless grapevines can be deWcit irrigated, increasing water use eYciency while maximizing yields.

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